The invention pertains to the art of current transformers and more particularly to a circuit providing a forced reset of a current transformer within a short time interval.
The invention is applicable to switching circuits, including power supplies, including resonant transition switching and, where core saturation of a current transformer can occur. It will be appreciated, however, that the invention has broader applications and can be advantageously employed in other environments and applications.
Resonant transition switching circuits are well known in the art. Such circuits are also known to include power factor correction circuitry which achieves boost regulation. A typical circuit of this type consists of an input in series with an inductor and a switch to connect one end of the inductor to circuit common. This arrangement provides an input rectified line through an inductor. When a switch of the switching circuit turns on, current builds within the inductor thereby storing energy in the inductor. When the switch turns off energy stored in the inductor, particularly I.sup.2 L energy, flows through a path including, for example, a free wheeling diode used to charge a bank capacitor.
In resonant transition switching circuits, such a boost regulator circuit employs resonant components including a resonant inductor and a resonant switch. The resonant components cooperate to minimize losses during switching time periods by storing energy that would have normally been lost in discharging a drain capacitance of a FET type switch, and storing this energy within the resonant inductor.
Resonant transition switching circuits are known to commonly include current transformers. However, a problem which exists and to which the subject application is directed, is that magnetic saturation of the current transformers can occur in switching circuits of this type since current in the current transformer can potentially flow for substantially one-hundred percent (100%) of a duty cycle. This situation exists since a duty cycle involves initially switching on a resonant switch and thereafter turning on a main switch. This type of arrangement causes a rise of current in the current transformer, starting at an initial time zero. Current initially builds in the current transformer due to current flow within the resonant circuitry and then due to the current flowing in the main circuit after the main switch has been turned on. Under these conditions, current builds in the resonant switch and then flattens out and decays, however, prior to a full decay of the current in the resonant switch the main switch begins building current and carries current until the end of the duty cycle. The duty cycle will approach 100%. Therefore, when investigating the state of a switching circuit, current will be seen as building in the main switch as current is falling in the resonant switch. However, the current transformer will see the sum of these currents. Due to this topology the current transformer will sense current for almost the entire duty cycle. When this situation occurs, the current transformer has a short time to reset.
In circuits which allow sufficient time between switching duty cycles, the current transformer will perform a natural reset. The reset is accomplished with I.sup.2 L energy stored in the current transformer shunt inductance during the "set" interval. However, in high frequency resonant transition switching circuits of the type contemplated for use with the subject invention, the energy stored for reset causes high "set" currents.
In order to maintain correct current transformer operation and to avoid high "set" currents which may cause distortion and current transformer saturation, it is necessary to provide for some sort of external current transformer reset. In view of this, to perform a reset it is necessary to provide the current transformer with a relatively large reset current pulse in a very short time period. For instance, if a switching circuit is used which has a power pulse time period or duty cycle of five (5) microseconds, the current transformer can be on for substantially this entire time period, and there may be only tens of nanoseconds in which to provide a reset pulse for the current transformer before a next power pulse will start. If the current transformer is not externally reset within the small window of time between power pulses, the reset energy required to be stored can take the current transformer into forward saturation.
However, while it is desired to reset a current transformer with a reset power pulse that is sufficient to move the current transformer from potential saturation to a lower flux level it is not desirable to provide an energy pulse, which resets the current transformer, but which also takes it into reverse saturation thereby undesirably storing energy which can be released as part of the set interval. This could be a detriment to overall circuit operation. Therefore, it is considered that if the energy pulse is either too large or applied for too long a period, unstable operation of the circuit can result.
It has, therefore, been deemed desirable to construct a circuit which provides a sufficient reset power pulse in a short time period that resets the current transformer to a desired flux level, but which does not drive the magnetization curve into reverse saturation.
Still further, the circuit should be easily constructed within existing switching circuit designs wherein its application resets the current transformer without affecting the main power pulses.